In recent years, LEDs are beginning to be used as a light source for a vehicle-mounted head lamp or a vehicle-mounted tail lamp. However, LEDs still have a low light-emitting efficiency, and, in order to make sure that LEDs used for a head lamp have a sufficient light emission quantity, supplied power of the same order as that to an electric-discharge head lamp is required while the head lamp which employs the LEDs and an electric power supply for lighting requires the same order of power consumption in the LEDs and the electric power supply for lighting as that of an electric-discharge head lamp. Therefore, at the present time, it is necessary to reduce both the power consumption of the LEDs and that of the lighting electric power supply from the viewpoint of both measures against heat generation in the LEDs and the lighting electric power supply, and energy saving. The same problem arises even in a case in which LEDs are used as a light source for a tail lamp.
Furthermore, people visually recognize the brightness of LEDs at the time of a peak current conducting through the LEDs as the brightness of the light source. Therefore, as a method of increasing the light emission quantity of LEDs visually with small lighting electric power, a lighting method of using the fact that a light source having a larger peak current is perceived as a brighter one, and, in a structure using LEDs or fluorescent display tubes for a display for displaying numbers, characters, and so on, alternately and repeatedly switching between conduction (lighting) of a large current pulse having a short time duration which exceeds a DC rated current through each segment (each light emitting element such as an LED or a fluorescent display tube) and nonconduction (lights-out) of the large current pulse at a high speed in such a way that the switching is not recognized visually as a flicker while maintaining the average power at rated power or smaller is generally used.
As a technology of carrying out such pulse lighting, a technology of conducting a pulsed current to LEDs for illumination to make them light up is described in the following related art references. For example, in patent reference 1, a technology of making variable the energy stored in a coil when a switching element in a step-up electric power supply is placed in an on state to acquire an arbitrary amount of output current for LED lighting is disclosed. In order to implement this technology, a device described in patent reference 1 uses an alternating current power supply as an electric power supply, averages the output current during a time period longer than the period of the alternating current power supply, and controls properly the current which is conducted to the switching element of the step-up electric power supply when the switching element is placed in the on state in such a way that the averaged output current has a target current value.
Furthermore, in patent reference 2, a technology of fixing the energy stored in a coil when a switching element in a step-up electric power supply is placed in an on state to a constant to acquire an arbitrary amount of output current for LED lighting is disclosed. A circuit described in patent reference 2 uses a direct current power supply of a portable device as an electric power supply, averages an output current of the electric power supply, and changes the ratio between the on and off switching time duration of the switching element of the step-up electric power, and the off duration of time that the switching element is held in the off state in such a way that the averaged output current has a target current value to control the switching element to make this switching element perform intermittently.